1. Field of the Invention
The present invention relates to methods and devices for the discovery and identification of new semiconductor materials as detector materials, and semiconductor materials which have been identified using these methods.
2. Description of the Related Art
The discovery of new semiconductors triggers new applications in fields as diverse as optoelectronics, sensors, detectors and power electronics. The exploration for new semiconductor materials increases the chances of finding a heavy-atom compound that can be grown as large crystals with little carrier trapping. It well may be that the current list of known compounds (e.g. HgI2, PbI2, CZT, TlBr, AlSb) does not contain the best possible materials. Furthermore new materials may become candidates for heavy-atom, ultra-fast, luminous semiconductor scintillators and allow radiative transition [e.g. donor band-acceptor CdS(In, Te)] and have a small band gap such as 200,000 photon/MeV limit.
While it is generally recognized that materials with a small band gap are semiconductors and those with a large band gap are insulators, there are notable exceptions, such as diamond. X-ray diffraction measurements have been made for over 100,000 crystalline materials and the results fill the Inorganic Crystal Structure and Powder Diffraction Databases, but bandgaps have been measured for only a small percentage of them. Even so, the bandgap alone is not a useful characteristic for identifying semiconductors because the bandgap range between 1.5 and 3.5 eV that contains many useful semiconductors also contains insulators. A temperature-dependent electrical conductivity is not useful for identifying semiconductors because only semiconductors with bandgaps below 1.5 eV exhibit appreciable thermal ionization at reasonable temperatures. Furthermore, temperature-dependent electrical conductivity is not useful because it is negligible in undoped semiconductors with band gaps above 1.5 eV but can be large for insulators that exhibit ionic conductivity. For example NaCl is not a semiconductor because holes are spontaneously trapped on the Cl2−Vk center but the electrical conductivity is high due to motion of the Cl− ions. Moreover, many insulators have large ionic conductivities that increase with temperature.
A successful semiconductor radiation detector material should have good stopping power, can be obtained as large crystals at low cost, have acceptable carrier mobilities and lifetimes, and operate at ambient temperatures. Despite the fact that available detector materials fall short of these goals, the list of candidate materials has not grown substantially during the past 25 years.
Many thousands of compounds can be prepared in crystalline form, but only a small fraction have been explored as detector materials. There is a need to identify semiconductor detector materials in the early stages of exploration, when samples are not available as single crystals, but only as crystalline powders. The band gaps can be estimated by measuring reflectance, but this alone does not determine which are semiconductors. The defining characteristic of a semiconductor (for circuits and nuclear detectors) is electron and hole mobility.
Thus, a semiconductor is a non-metal in which electron and hole carriers are mobile, and an insulator is a non-metal in which electrons or holes spontaneously trap. To function as a semiconductor detector both charge carriers (electrons and holes) must be mobile in an electric field. If either charge carrier is trapped by the spontaneous formation of a defect (e.g. a Vk center) in the material, the material will become polarized and ineffective as a detector. Herein are described methods and devices for the discovery and identification of new semiconductor materials based on the mobility of internally created electrons and holes.